1. Field of the Invention
The present invention relates to a digital-to-analog converter with high driving capability, and more particularly, to a resistor string digital-to-analog converter utilizing current source circuits for providing driving currents to improve response speed and reduce effects of parasitic resistors.
2. Description of the Prior Art
A digital-to-analog converter is a common circuit element in various electronic devices, and can generate an analog output voltage for application to back-end circuits according to a digital input value. Many techniques are available for realizing the digital-to-analog converter, among which a resistor string (R-string) digital-to-analog converter is a direct realization method that has a simple circuit structure as well.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art R-string digital-to-analog converter 10. The digital-to-analog converter 10 includes a voltage generator 11, a voltage division circuit 12, a decoding unit 13, and a switch circuit 14. The voltage generator 11 includes a first voltage source 110 and a second voltage source 115, which are respectively utilized for generating a first voltage V1 and a second voltage V2. The voltage division circuit 12 includes a first terminal 120, a second terminal 125, resistors R1-Rn, and output terminals OP1-OPn. The first terminal 120 and the second terminal 125 are respectively coupled to the first voltage source 110 and the second voltage source 115; the resistors R1-Rn are coupled in series, and are coupled between the first terminal 120 and the second terminal 125; and the output terminals OP1-OPn, respectively coupled between adjacent resistors of the resistors R1-Rn, are utilized for outputting reference voltages Vref1-Vrefn. Thus, the voltage division circuit 12 performs voltage division by the resistors R1-Rn to generate and output the reference voltages Vref1-Vrefn to the output terminals OP1-OPn according to the first voltage V1 and the second voltage V2. The decoding unit 13 is utilized for receiving a digital input value and generating a control signal Ctr, accordingly. The switch circuit 14 is coupled to the voltage division circuit 12 and the decoding unit 13, and is utilized for switching to output one of the reference voltages Vref1-Vrefn according to the control signal Ctr outputted by the decoding unit 13. In order to meet requirements for ideal voltage sources, the first voltage source 110 and the second voltage source 115 can be realized by negative-feedback operational amplifier circuits, as shown in FIG. 2 and FIG. 3, respectively.
Therefore, using the resistor string of the voltage division circuit 12, the prior art digital-to-analog converter 10 can perform voltage division to generate the demanded reference voltages Vref1-Vrefn. And, by utilizing the decoding unit 13 for decoding to output the control signal Ctr according to the input digital signal, the switch circuit 14 can then switch to output an analog voltage corresponding to the input digital signal correctly.
The R-string digital-to-analog converter is most often used today to generate a single analog output voltage, and thus output loads of each voltage division node of the resistor string (i.e. the output terminals OP1-OPn) are very low. However, for applications in some specific fields, such as in a source driver of a liquid crystal display, multiple analog voltages often need to be outputted at the same time. Thus, more than one of the switch circuits 14 need to be coupled to the voltage division circuit 13, with a result that the output loads of the voltage division circuit 13 are increased greatly. In general, as the output voltage of the digital-to-analog converter 10 varies, the first voltage source 110 and the second voltage source 115 also vary to output the corresponding driving currents according to variation of the output loads. However, some response time is needed when the operational amplifiers of the first voltage source 110 and the second voltage source 115 react. Especially in applications with larger output loads, the range of the driving currents is too large, so that the voltage source circuits realized by the negative-feedback operational amplifier circuits may have poor response speed.
In addition, when laying out the R-string digital-to-analog converter, there exist parasitic resistors PR1 and PR2 respectively between the voltage sources 110 and 115 and the voltage division circuit 12, as shown in FIG. 1. In this case, part of the voltages generated by the voltage sources 110 and 115 may be lost in the parasitic resistors PR1 and PR2, and thus errors in the reference voltages Vref1-Vrefn outputted by the voltage division circuit 12 occur, which further influences accuracy of the outputted analog voltages.